Methods
Selection criteria and data collection
We performed a systematic search of SciVerse Scopus, PubMed MEDLINE, and
Web of Science databases from inception of each database up to the 5th
December 2022 using the following boolean search commands:
[“biodiversity”] AND [“allerg*”] OR [“asthma”] AND
[“environment*”] AND [“microbio*”]. The methodology used in
this systematic review and meta-analysis followed the PRISMA reporting
standards. The protocol was published in the PROSPERO (registration
number: CRD42022381725) in December 2022
(https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42022381725).
The full search strategy can be found in the online supplement (p 6).
A study was included in the systematic review and meta-analysis if (i)
it was an original randomized controlled trial or an epidemiologic study
(cohort, case-control or cross-sectional study); (ii) provided data on
the relations between inner and/or outer layers of biodiversity and the
risk of asthma, wheezing, and/or allergic sensitization, (iii) included
an adequate definition and/or indexes of biodiversity and the main
health outcomes (asthma, wheezing and allergic sensitization). Studies
reporting only asthma exacerbation as the outcome or assessing the
effect of antibiotics, medication or treatment were excluded. Studies
assessing only the effect of endotoxins, fungi or virus, diet,
breastfeeding, mode of delivery, use of
probiotics/prebiotics/symbiotics, pet exposure, exposure to farm, rural,
or urban environments, and to natural and green spaces were also
excluded.
Exposure
The exposures of interest were the outer and/or inner layer
biodiversity.6 The outer layer is dependent on the
environment we live in (including soil, natural waters, plants, and
animals). The inner layer inhabits human body (including gut, skin, and
airways) and is influenced by colonization from the outer
layer.6 The outer layer biodiversity was defined based
on the environmental microbiota (including dust or soil) or on a score
of environmental biodiversity (e.g., species richness index, land-use
gradient, and plant diversity). The inner layer biodiversity was defined
based on the diversity of human inner microbiota in the skin, stool,
urine, and airway samples (including nasal, oropharyngeal,
nasopharyngeal, and/or throat) (online supplement p 6).
Outcomes
The primary outcomes included asthma and the symptom wheezing. Different
asthma definitions were considered for the inclusion of studies, such as
the medical diagnosis, use of asthma medications, or the GINA-based
definition.12 The secondary outcome was allergic
sensitization which was defined based on a positive response to IgE
antibodies for specific allergens, or a positive skin prick test (SPT).
Data extraction
Three reviewers (IP, NS, BH) independently performed the initial
screening by the title and the abstract. After this initial screening,
full articles were reviewed by two independent reviewers (TH, AR) and
those fulfilling the inclusion criteria were selected for data
extraction. When a conflict in assessment arose at any stage of the
review, a consensus between the original and two additional reviewers
(TH, AR) was required to achieve the resolution. Additional description
of the data extraction process is presented in the online supplement (p
6).
Quality assessment and publication
bias
The risk of bias was independently assessed by two reviewers (IP and NS)
applying the Newcastle-Ottawa Scale (NOS) for cohort and case-control
studies.13 An adapted version of the NOS developed by
Herzog et al. 14 was used for cross-sectional studies.
The results from the NOS were
translated into the Agency for Health Research and Quality standards,
and applying these the studies were classified as good, fair or
poor.15
Statistical methods
A study was included in the meta-analysis if (i) it used a priori
recognised measure to assess biodiversity (Shannon diversity index
and/or bacterial richness) and (ii) it analysed biodiversity as a
continuous variable. Meta-analysis applying the random-effects model was
performed on the available data applying the R software, Version
1.4.1106 (dmetar package) and SAS software. To evaluate the effect of
biodiversity, the data were summarised as the standardized mean
difference (SMD) or the effect estimate (EE) with 95% confidence
interval applying the Hedges method and the restricted maximum
likelihood (REML) estimator, respectively. The magnitude of
heterogeneity between the included studies was estimated using the
Higgins I2 statistic and τ2.
Publication bias was assessed through visual examination of a funnel
plot and by applying the Egger’s test. Sensitivity analysis was
performed according to the risk of bias/quality of evidence by comparing
the summary effect estimates obtained by excluding studies with a high
risk of bias with those studies considered as being at low and moderate
risk of bias.